Added support for taking screenshot during Notebook publish to Gallery (#108)

* Added support for taking screenshot

- Screenshot is taken using html2canvas package
- Converted to base 64 and uploaded to metadata
- For Using first display output
  - Notebok object is passed instead of string, to publish pane
  - The first cell with output present is parsed out
  - The dom is also parsed to get corresponding div element to take screenshot of the first output

* fixed bug

* Addressed PR comments

- FIxed bug that didn't capture screenshot when mutiple notebook tabs are opened

* removed unnecessary dependencies

* fixed compile issues

* more edits

src/Explorer/Notebook/NotebookComponent/actions.ts

@@ -84,3 +84,22 @@ export const traceNotebookTelemetry = (payload: {
     payload
   };
 };
+
+export const UPDATE_NOTEBOOK_PARENT_DOM_ELTS = "UPDATE_NOTEBOOK_PARENT_DOM_ELTS";
+export interface UpdateNotebookParentDomEltAction {
+  type: "UPDATE_NOTEBOOK_PARENT_DOM_ELTS";
+  payload: {
+    contentRef: ContentRef;
+    parentElt: HTMLElement;
+  };
+}
+
+export const UpdateNotebookParentDomElt = (payload: {
+  contentRef: ContentRef;
+  parentElt: HTMLElement;
+}): UpdateNotebookParentDomEltAction => {
+  return {
+    type: UPDATE_NOTEBOOK_PARENT_DOM_ELTS,
+    payload
+  };
+};